The concentration of CO2 in the atmosphere has increased over the past 200 years and is expected to continue rising in the next 50 years at a rate of 3 ppm·year−1. This increase has led to a decrease in seawater pH that has changed inorganic carbon chemical speciation, increasing the dissolved HC O 3 − . Posidonia oceanica is a marine angiosperm that uses HC O 3 − as an inorganic carbon source for photosynthesis. An important side effect of the direct uptake of HC O 3 − is the diminution of cytosolic Cl− (Cl−c) in mesophyll leaf cells due to the efflux through anion channels and, probably, to intracellular compartmentalization. Since anion channels are also permeable to N O 3 − we hypothesize that high HC O 3 − , or even CO2, would also promote a decrease of cytosolic N O 3 − ( N O 3 − c ). In this work we have used N O 3 − - and Cl−-selective microelectrodes for the continuous monitoring of the cytosolic concentration of both anions in P. oceanica leaf cells. Under light conditions, mesophyll leaf cells showed a N O 3 − c of 5.7 ± 0.2 mM, which rose up to 7.2 ± 0.6 mM after 30 min in the dark. The enrichment of natural seawater (NSW) with 3 mM NaHCO3 caused both a N O 3 − c decrease of 1 ± 0.04 mM and a Cl c − decrease of 3.5 ± 0.1 mM. The saturation of NSW with 1000 ppm CO2 also produced a diminution of the N O 3 − c , but lower (0.4 ± 0.07 mM). These results indicate that the rise of dissolved inorganic carbon ( HC O 3 − or CO2) in NSW would have an effect on the cytosolic anion homeostasis mechanisms in P. oceanica leaf cells. In the presence of 0.1 mM ethoxyzolamide, the plasma membrane-permeable carbonic anhydrase inhibitor, the CO2-induced cytosolic N O 3 − diminution was much lower (0.1 ± 0.08 mM), pointing to HC O 3 − as the inorganic carbon species that causes the cytosolic N O 3 − leak. The incubation of P. oceanica leaf pieces in 3 mM HC O 3 − -enriched NSW triggered a short-term external N O 3 − net concentration increase consistent with the N O 3 − c leak. As a consequence, the cytosolic N O 3 − diminution induced in high inorganic carbon could result in both the decrease of metabolic N flux and the concomitant biomass N impoverishment in P. oceanica and, probably, in other aquatic plants.